With the trend toward weight and size reduction in electrical products, lithium secondary batteries which are lightweight nonaqueous-electrolyte secondary batteries having a high energy density are used in a wide range of fields. A lithium secondary battery is generally constituted mainly of: a positive electrode obtained by forming an active-material layer containing a positive-electrode active material such as a lithium compound represented by lithium cobalt oxide on a current collector; a negative electrode obtained by forming an active-material layer containing a negative-electrode active material such as a carbon material capable of occluding/releasing lithium, which is represented by graphite or the like, on a current collector; a nonaqueous electrolyte solution prepared by dissolving an electrolyte such as a lithium salt, e.g., LiPF6, usually in an aprotic nonaqueous solvent; and a separator comprising a porous polymer film.
It is known that in such secondary batteries, to heighten the packing density of an electrode active material is effective in improving battery performances including battery capacity. For example, patent document 1 describes a technique in which a negative-electrode active material such as, e.g., flaky graphite is rounded by a mechanochemical action to thereby enable the active material to be packed at a higher density and attain a higher capacity. Patent document 2 describes a technique in which lithium nickel cobalt oxide positive-electrode active-material particles whose secondary particles are spherical or ellipsoidal are used to improve high-load characteristics and low-temperature high-rate discharge characteristics.
Incidentally, the separators for use in lithium secondary batteries are required to satisfy the following and other requirements: not to inhibit ionic conduction between the two electrodes; to be capable of holding electrolyte solutions; and to have solvent resistance to electrolyte solutions. Porous polymer films made of thermoplastic resins such as polyethylene and polypropylene are mainly used. The following techniques have hitherto been known as processes for producing those porous polymer films.
(1) An extraction method which comprises adding to a polymeric material a plasticizer capable of being easily extracted/removed in a later step, molding the mixture, and thereafter removing the plasticizer with an appropriate solvent to make the porous structure (patent document 3).
(2) A stretching method which comprises molding a crystalline polymeric material and then selectively stretching the amorphous parts, which are structurally weak, to thereby form micropores (patent document 4).
(3) An interfacial separation method which comprises adding a filler to a polymeric material, molding the mixture, and thereafter conducting a stretching operation to cause separation at the interface between the polymeric material and the filler and thereby form microvoids (patent document 5).
However, the extraction method (1) necessitates treatment of a large amount of a waste liquid and hence has problems concerning both environment and profitability. In addition, since film shrinkage occurs in the extraction step and this makes it difficult to obtain an even film, this method further has a problem concerning productivity such as yield. The stretching method (2) necessitates a long-term heat treatment because a pore diameter distribution is regulated by regulating the crystalline phase/amorphous phase structure before stretching. This method hence has a problem concerning productivity.
In contrast, the interfacial separation method (3) generates almost no waste liquid or the like and is an excellent method from the standpoints of both environment and profitability. Furthermore, since separation at the interface between the polymeric material and the filler can be easily caused by a stretching operation, a porous film can be obtained without necessitating a pretreatment such as a heat treatment. This method hence is a technique excellent also from the standpoint of productivity.
However, it has commonly been thought that the separators containing a filler are poor in the property of coming into tight contact with electrodes because of the presence of the filler projecting from the surface to cause unevenness in interelectrode resistance due to uneven electrode spacings and are apt to generate lithium dendrites or the like and inferior in safety. Because of this, there have been no cases where the filler-containing separator produced by the method (3) described above has been put to practical use.
In patent document 1, which was cited above, the term separator is merely given in paragraph [0048]. In patent document 2 also, separators are described in paragraph [0041] only as microporous films or nonwoven fabrics made of polypropylene, polyethylene, or a copolymer. In general, these microporous polyolefin separators are industrially produced by the extraction method (1). or stretching method (2) described above. In addition, patent documents 1 and 2 include no statement concerning influences of the separators in the case where a specific active material such as those shown above is used.
Furthermore, it has commonly been thought that the separators containing a filler are poor in the property of coming into tight contact with electrodes because of the presence of the filler projecting from the surface to cause unevenness in interelectrode resistance due to uneven electrode spacings and are apt to generate lithium dendrites or the like and inferior in safety. Because of this, there have been no cases where the filler-containing separator produced by the method (3) described above has been put to practical use.